remove $wgAutocreatePolicy global and all references to it

## Summary ##

This is a fairly simple clean-up on along the way of Helios project. This thing (automatic creation of local accounts) has been removed in MediaWiki 1.22. We are not going to need it once we have the Helios service operational. In fact, we are not using it today. I decided to remove it.

## Changes ##
* `includes/DefaultSettings.php` - this is where the default value has been set; `login` is MediaWiki's original default.

* `includes/wikia/Wikia.php` - the `setupAfterCache()` method is executed very early and the value of `wgAutocreatePolicy` is changed to `view`. Always (considering `$wgExternalAuthType`). We could have changed the default value.

* `includes/User.php` - considering the above, the removed condition always evaluates to `true` so the code inside is always executed. Let's clearly state it - in our case it is unconditional. Also, note that this is our own change to core code.

* `includes/specials/SpecialUserlogin.php` - see the per-line comments.